Bottom Line:
A small GTPase Arf6 primarily regulates recycling of plasma membrane components.A series of our studies revealed that for activation of the invasion pathway of EGFR, it is prerequisite that Arf6 and AMAP1 both are highly overexpressed, and that EGFR is activated by ligands.Microenvironments have been highly implicated in the malignancy of mammary tumors.

ABSTRACTTumors are tissue-specific diseases, and their mechanisms of invasion and metastasis are highly diverse. In breast cancer, biomarkers that specifically correlate with the invasive phenotypes have not been clearly identified. A small GTPase Arf6 primarily regulates recycling of plasma membrane components. We have shown that Arf6 and its effector AMAP1 (DDEF1, DEF1, ASAP1 and centaurin beta4) are abnormally overexpressed in some breast cancers and used for their invasion and metastasis. Overexpression of these proteins is independent of the transcriptional upregulation of their genes, and occurs only in highly malignant breast cancer cells. We recently identified GEP100 (BRAG2) to be responsible for the Arf6 activation to induce invasion and metastasis, by directly binding to ligand-activated epidermal growth factor receptor (EGFR). A series of our studies revealed that for activation of the invasion pathway of EGFR, it is prerequisite that Arf6 and AMAP1 both are highly overexpressed, and that EGFR is activated by ligands. Pathological analyses indicate that a significant large population of human ductal cancers may utilize the EGFR-GEP100-Arf6-AMAP1 pathway for their malignancy. Microenvironments have been highly implicated in the malignancy of mammary tumors. Our results reveal an aspect of the precise molecular mechanisms of some breast cancers, in which full invasiveness is not acquired just by intracellular alterations of cancer cells, but extracellular factors from microenvironments may also be necessary. Possible translation of our knowledge to cancer therapeutics will also be discussed.

fig02: The secondary structure of the 5’-untranslated regions of Arf6 mRNA and AMAP1 mRNA.

Mentions:
Gene expression profiling analyses have not identified the Arf6 gene as being overexpressed in invasive ductal carcinomas (IDCs). Our analysis on cultured breast cancer cell lines has instead revealed that all of the breast cancer cell lines we examined express comparable levels of Arf6 mRNA, regardless of their invasiveness (20). HMECs also expressed high levels of Arf6 mRNA, as observed in highly invasive breast cancer cell lines. Thus, the selective overexpression of the Arf6 protein in highly invasive breast cancer cells appears to be independent of the enhanced transcription of the Arf6 gene, but may be because of its posttranscriptional regulation. Consistent with this notion, the 5’-untranslated region of the Arf6 mRNA is very long and possesses a very complicated secondary structure with a very high level of free-energy change [http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi? view=graph&val=NT_026437. 11&_gene=ARF6] (Figure 2). mTOR is one of the central kinases regulating 5’-cap-dependent translational control (26), and is frequently overexpressed in breast tumors (27). We however have preliminary results indicating that Arf6 mRNA is not under the control of mTOR (OBI, Osaka).

fig02: The secondary structure of the 5’-untranslated regions of Arf6 mRNA and AMAP1 mRNA.

Mentions:
Gene expression profiling analyses have not identified the Arf6 gene as being overexpressed in invasive ductal carcinomas (IDCs). Our analysis on cultured breast cancer cell lines has instead revealed that all of the breast cancer cell lines we examined express comparable levels of Arf6 mRNA, regardless of their invasiveness (20). HMECs also expressed high levels of Arf6 mRNA, as observed in highly invasive breast cancer cell lines. Thus, the selective overexpression of the Arf6 protein in highly invasive breast cancer cells appears to be independent of the enhanced transcription of the Arf6 gene, but may be because of its posttranscriptional regulation. Consistent with this notion, the 5’-untranslated region of the Arf6 mRNA is very long and possesses a very complicated secondary structure with a very high level of free-energy change [http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi? view=graph&val=NT_026437. 11&_gene=ARF6] (Figure 2). mTOR is one of the central kinases regulating 5’-cap-dependent translational control (26), and is frequently overexpressed in breast tumors (27). We however have preliminary results indicating that Arf6 mRNA is not under the control of mTOR (OBI, Osaka).

Bottom Line:
A small GTPase Arf6 primarily regulates recycling of plasma membrane components.A series of our studies revealed that for activation of the invasion pathway of EGFR, it is prerequisite that Arf6 and AMAP1 both are highly overexpressed, and that EGFR is activated by ligands.Microenvironments have been highly implicated in the malignancy of mammary tumors.

ABSTRACTTumors are tissue-specific diseases, and their mechanisms of invasion and metastasis are highly diverse. In breast cancer, biomarkers that specifically correlate with the invasive phenotypes have not been clearly identified. A small GTPase Arf6 primarily regulates recycling of plasma membrane components. We have shown that Arf6 and its effector AMAP1 (DDEF1, DEF1, ASAP1 and centaurin beta4) are abnormally overexpressed in some breast cancers and used for their invasion and metastasis. Overexpression of these proteins is independent of the transcriptional upregulation of their genes, and occurs only in highly malignant breast cancer cells. We recently identified GEP100 (BRAG2) to be responsible for the Arf6 activation to induce invasion and metastasis, by directly binding to ligand-activated epidermal growth factor receptor (EGFR). A series of our studies revealed that for activation of the invasion pathway of EGFR, it is prerequisite that Arf6 and AMAP1 both are highly overexpressed, and that EGFR is activated by ligands. Pathological analyses indicate that a significant large population of human ductal cancers may utilize the EGFR-GEP100-Arf6-AMAP1 pathway for their malignancy. Microenvironments have been highly implicated in the malignancy of mammary tumors. Our results reveal an aspect of the precise molecular mechanisms of some breast cancers, in which full invasiveness is not acquired just by intracellular alterations of cancer cells, but extracellular factors from microenvironments may also be necessary. Possible translation of our knowledge to cancer therapeutics will also be discussed.